Garbage inventories do not affect ancestry queries, and removal is
considerably more expensive as there is no separate versioned file for
them, so they are not cleaned. In short it is currently a no-op.

In future this may be a good place to hook in annotation cache checking,
index recreation etc.

This removes them from the memory index, saves the in-memory index
which makes the newly reconciled pack visible and hides the packs to be
discarded, and finally renames the packs being discarded into the
obsolete packs directory.